Microfabrication method for optical components

ABSTRACT

A ferrule mold having a reverse-image of a through-hole array for optical fibers is formed. A non-polymeric ferrule material is deposited in the reverse-image mold, followed by removing the mold to create a multi-fiber connector ferrule having at least two fiber through-holes. An optical fiber is inserted in each through-hole until each fiber endface is positioned approximately even with a connection surface of the ferrule. A fiber recess for each of the optical fibers is formed such that each fiber is recessed from the multi-fiber ferrule connection surface by a distance of at least 0.1 micron. The recess may be formed by differential polishing of the non-polymeric ferrule and endfaces of the optical fibers. Alternatively, a layer of spacer material may be deposited over the multi-fiber ferrule connection surface. An antireflection coating is deposited over the ferrule connection surface and ends of the recessed fibers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. patent application Ser. No.16/505,673, filed Jul. 8, 2019, provisional application Ser. No.61/579,017, entitled “Non-Contact Optical Fiber Connector”, and filed onDec. 22, 2011, and U.S. patent application Ser. No. 13/725,087, filedDec. 21, 2012, the disclosures of which are incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates to a microfabrication method using amicrofabrication mold, material deposition, material removal, etching,and coating to create an optical component such as a multi-fiber opticalfiber connector.

BACKGROUND

Multi-fiber optical connectors are widely used by telecommunicationsystem operators for various applications. Multi-fiber connectors arepredominantly MPO fiber connectors. Commercially available MPO fiberconnectors include fiber counts of, for example, 8, 12, 24, 48, or 72fibers. For single mode (SM) fiber connectors which has much smallerfiber alignment tolerance than multimode fibers due to its much smallerfiber core, the most common fiber count of SM MPO connectors is 12fibers today. Higher fiber count SM MPO connectors such as 24 or 48fibers are also possible, although their optical performancedeteriorates seriously compared to 12 fiber SM MPO connectors.

In part, the deteriorating performance with higher fiber counts isbecause conventional connectors are made with traditional polymermolding techniques; these molding techniques cannot achieve the precisealignment required. For example, physical pins must be positionedaccurately in the mold to form the fiber holes and guide holes. However,it is very difficult to use molding to achieve a higher count2-dimensional (2D) fiber holes accurately.

To satisfy the increasing demand of massive data transfer, components inoptical communication systems are being developed to be miniaturized andhighly integrated. Particularly, integrating large numbers of SM fibers(such as 144 or 576) on a single fiber connector is a long-felt need.However, great challenges lie in the fabrication of such fiberconnectors.

There have been several approaches to make multi-fiber connectors withhigh-density 2D fiber arrays. In the following, each approach isreviewed.

1) Traditional MPO Multi-Fiber Connector

MPO connectors use MT connector ferrules to align the fibers. The MTconnector ferrules for SM fibers typically have one linear array of 12or 8 fiber holes, and two large guide holes for guide pin alignment.

MT connector ferrules are plastic parts made by plastic moldingprocesses. Because fiber holes and guide holes are formed by moldingplastics around physical pins in the mold, when there is a high count 2Darray of physical pins, it is very difficult to position these pinsaccurately and replicate the mold faithfully.

Although a 72-fiber multimode MPO connector in a 2D configuration (12×6)has been introduced by some manufacturers, its geometrical precision offiber holes and guide holes is far inferior to 12-fiber multimode MPOconnectors. In addition, a SM fiber version of this connector isunavailable.

The operating principle of traditional MPO connectors is physicalcontact. Fibers are polished so that they protrude from the surroundingMT ferrule surface, to ensure uniform physical contact of every fibersurface. Higher fiber count MPO connectors not only have problems withSM fiber alignment tolerance, they also suffer from problems such aspoor physical contact and sensitivity to dust and contaminants.

2) MXC Connector

In 2013, MXC connectors with 64 fibers using lenses were announced byCorning, Intel, and US Conec. While the lens-based design allows thisconnector to operate without physical contact and ensure much morerobust connection, insertion loss is high (0.8 dB insertion loss) evenfor multimode fiber. Furthermore, MXC connectors are not availabilityfor SM fiber.

The fiber ferrules in the MXC connectors are made by molding plastics,so these connectors suffer the same limitations as MPO connectorferrules.

A previous approach to non-contact fiber connectors is described by theinventor of the present application in U.S. patent application Ser. No.13/725,087, filed Dec. 21, 2012, the disclosure of which is incorporatedby reference herein.

Thus, there is a need in the art for improved manufacturing techniquesfor forming high-precision multi-fiber optical fiber connectors,especially for SM fibers. Such high precision multi-fiber optical fiberconnectors could be used to terminate the large number of optical fibersneeded in modern data centers and telecommunication systems.

SUMMARY OF THE INVENTION

The present invention provides a microfabrication method formanufacturing a high-density, recessed-fiber, multi-fiber opticalconnector ferrule, capable of providing sub-micron fiber alignmentprecision suitable for SM optical fibers. The method includesmicrofabricating a ferrule mold having a reverse-image of a through-holearray for optical fibers and guide holes. The through-hole array has atleast four through-holes, including two guide holes and two fiber holes.Each guide hole is dimensioned and configured to hold and align a guidepin. Each fiber hole is dimensioned and configured to hold and align anoptical fiber. Guide holes facilitate passive alignment between matingoptical connectors.

A non-polymeric ferrule material is deposited in said ferrule mold,followed by removing the ferrule mold to create a non-polymericmulti-fiber connector ferrule.

An optical fiber is inserted to extend through each of the fiber holesand affixed by a suitable adhesive. Each optical fiber is polished toform an endface, which is approximately level with the top surface ofthe multi-fiber connector ferrule but is slightly recessed from said topsurface by a distance of at least approximately 0.1 micron. The recessmay be formed by differential polishing of the non-polymeric ferrule andfiber endfaces. Alternatively, a layer of spacer material may bedeposited over said top surface of the multi-fiber connector ferrule.

An antireflection coating is deposited such that it covers said topsurface of the multi-fiber connector ferrule and the endfaces of therecessed fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a and FIG. 1b depict perspective and cross-sectional views of amulti-fiber optical connector ferrule 10 made by the method of thepresent invention.

FIG. 2a shows a silicon wafer bonded to a support wafer.

FIG. 2b shows the silicon wafer structure of FIG. 2a with a photoresistlayer coated on its top surface.

FIG. 2c shows a photomask which contains guide holes, fiber holes andoptional ferrule chip wall.

FIG. 2d shows photolithography performed with the photomask of FIG. 2 c.

FIG. 2e shows the structure of FIG. 2b after the photoresist developmentstep.

FIG. 2f shows the structure of FIG. 2e after dry etching step.

FIG. 2g shows the completed silicon ferrule mold.

FIG. 2h is a perspective view of the silicon ferrule mold.

FIG. 2i shows the silicon ferrule mold after the electroplating step iscompleted.

FIG. 2j shows the electroplated silicon ferrule mold after the polishingstep is completed.

FIG. 2k shows a finished ferrule chip.

FIG. 2l further depicts the finished ferrule chip.

FIG. 3a shows a multi-fiber connector ferrule including a ferrule chipand a ferrule pedestal.

FIG. 3b shows the multi-fiber connector ferrule after fibers areinserted through the fiber through-holes and permanently affixed withepoxy.

FIG. 3c shows the multi-fiber connector ferrule after the polishingstep, showing recessed fiber endfaces.

FIG. 3d shows the multi-fiber connector ferrule 10 after theantireflection coating.

FIG. 3e shows a male multi-fiber connector ferrule with guide pins inplace.

FIG. 3f is a perspective view of the male multi-fiber connector ferrule.

FIG. 3g shows the female and male multi-fiber connector ferrules matedtogether.

FIG. 4a shows a multi-fiber connector ferrule with no tilt angle, withfibers inserted.

FIG. 4b shows the female multi-fiber connector ferrule after thepolishing step, ready to receive guide pins.

FIG. 5 shows top view of a ferrule chip with modified guide holes withdust channels.

FIG. 6 shows top view of a ferrule chip with modified guide holes whichhave spring-loaded sidewalls.

FIG. 7 shows top view of a female ferrule chip with modified guideholes, where one guide hole is round while another guide hole has anelongated profile.

FIG. 8 is a cross sectional view showing a non-contact optical fiberconnector component according to a further embodiment of the presentinvention.

FIG. 9 shows a pair of such non-contact fiber connector components asshown in FIG. 8 mated together.

FIGS. 10(A) and 10(B) are contour plots of the recessed fiber surfacesof the non-contact optical fiber connector, as measured by a commercialfiber optic interferometer.

FIG. 11 is a cross sectional view showing another embodiment of thenon-contact optical fiber connector component according to the presentinvention.

FIG. 12 is a schematic drawing of a generic non-contact optical fiberconnector with a splice-on connector construction.

FIG. 13 is a schematic drawing of a sample holder for AR coating manynon-contact fiber connector components of the type in FIG. 8simultaneously.

FIG. 14 is a plan view of a non-contact multi-fiber connector pairaccording to an embodiment of this invention.

DETAILED DESCRIPTION

Turning to the drawings in detail, FIGS. 1a-1b depict perspective andcross-sectional views of a non-contact multi-fiber connector ferrule 10formed by the method of the present invention.

The non-contact multi-fiber connector ferrule 10 includes a ferrule chip20 and a ferrule pedestal 30. The ferrule chip 20 is significant becauseit provides the very high precision of this non-contact multi-fiberconnector ferrule 10.

The ferrule chip 20 has at least four through holes, including two guideholes 35 and two fiber holes 25. Each guide hole 35 is dimensioned andconfigured to hold and align a guide pin. Each fiber hole 25 isdimensioned and configured to hold and align an optical fiber. Guideholes 35 facilitate passive alignment between mating multi-fiber opticalfiber connectors.

While twenty-five fiber holes (5×5 array) are depicted for clarity ofpresentation, it is understood that substantially larger numbers offiber holes may be formed in the multi-fiber optical connectors of thepresent invention. For example, a 2D array of 12×12 fiber holes or 24×24fiber holes may also be formed in the multi-fiber optical connectors.Those of ordinary skill in the optical connector art will appreciatethat an arbitrary number of fiber holes in arbitrary configurations maybe formed using the disclosed techniques.

Inserted in each fiber hole 25 is an optical fiber 40 which may be asingle mode (SM) or multimode optical fiber. The precisemicrofabrication techniques used to make the ferrule chip create aconnector where the fiber alignment precision is sufficient for thesmaller core of the SM optical fibers. As seen in FIG. 1b each fiberendface 45 is recessed from a ferrule contact surface 50 of ferrule chip20. An antireflection (AR) coating 55 is deposited over each fiberendface 45 and on the ferrule contact surface 50. Guide holes 35 areoptionally included to facilitate passive alignment between matingoptical connectors.

To manufacture the ferrule chip 20, a ferrule mold is microfabricatedhaving “chimney-like” pillars for the fiber holes and guide holes, asdepicted in the example microfabrication process of FIGS. 2a-2l ,including the steps of mask preparation, photolithography, dry etching,metal electroplating and demolding.

In the example of FIGS. 2a-2l , the central axis of the fiber hole isslightly tilted with respect to the surface normal of the ferrule chip.In this exemplary embodiment, the tilt angle is selected to be 6degrees. The tilt angle is selected to minimize return loss for theoptical fiber connections. However, it is understood that themicrofabrication process depicted in FIGS. 2a-2l may also be used toform other angles or multi-fiber optical connectors with no angle.

The microfabrication process of FIGS. 2a-2l are performed using siliconas the mold material to form the ferrule chip 20. However, othermaterials may be used to form the mold material such as photoresist andother polymeric materials. Other semiconductor materials such asgermanium may also be used. Metals may also be used. In short, anymaterial that is susceptible to microfabrication techniques and can beformed with submicron accuracy may be used as the mold material to formferrule chip 20.

Wafer Bonding

Referring to FIG. 2a , there is a silicon wafer 100 that serves as thesubstrate for microfabrication, and another silicon wafer 102 thatserves as an optional support substrate. The two wafers 100 and 102 arebonded together and there is a metal layer 104 between the two siliconwafers for the subsequent electroplating step. The metal layer 104 ispreferably a thick layer of metal such as tin, aluminum, gold, or alloysthereof.

Photoresist Coating

In FIG. 2b , the silicon wafer 100 is shown coated with a photoresistlayer 106 on its top surface. The photoresist may be either organic orinorganic, positive or negative depending on the selected mask design.It may be applied by any appropriate techniques, including but notlimited to dipping, spraying, spinning or vapor deposition. In anexemplary embodiment, a spin-coated 5740 Novolak photoresist with a 2micron thick layer is used as layer 106.

Photomask

A photolithographic mask 200 is used for microfabrication. Referring toFIG. 2c , the photomask includes designed patterns of guide holes 204,an array of fiber holes 206 and optionally ferrule chip wall 202. Fiberholes and guide holes can take other forms than round hole.

Mask 200 assumes a positive photoresist, so that exposed photoresist isremoved in a subsequent development step.

UV Exposure

As shown in FIG. 2d , photolithography is performed with the photomask200. UV radiation 302 (or other selected radiation, including x-rays,laser radiation, etc., depending upon the selected photoresist)replicates the photomask pattern on the photoresist 106.

Photoresist Development

After photoresist development (involving removal of the exposedphotoresist), the pattern shown in FIG. 2e is formed. Areas intended tobe guide holes and fiber holes are protected by photoresist 304, whichserves as an etch barrier during subsequent processing.

Dry Etching

The silicon wafer 100 is etched in a dry etching process, as shown inFIG. 2f . In a preferred embodiment, etching should be done using a DRIE(deep reactive ion etching) process or other microfabrication techniqueswhich can provide sub-micron accuracy.

Dry etching plasma 308 will etch through the silicon wafer 100 from thetop surface until it reaches an etch stop layer, and metal layer 104 isexposed.

In a preferred embodiment, the wafer should be tilted with a small tiltangle with respect to the dry etching direction. The preferred tiltangle is 6 degree.

Stripping Photoresist

After the dry etching step, the remaining photoresist 304 should bestripped, so that a silicon ferrule mold is completed, including pillarsfor fiber holes 312 and pillars for guide holes 310, and a ferrule chipwall 314, as shown in FIG. 2g (side view). A perspective view of thesilicon ferrule mold is shown in FIG. 2h . In the preferred embodiment,the fabricated pillars 312 and 310 should have slightly enlarged bottom.

The structure depicted in FIG. 2g will serve as a mold for the ferrulechip as seen in the subsequent processing steps.

The ferrule mold can be formed using any microfabrication method whichcan provide sub-micron accuracy, including but not limited to LIGA, UVLIGA, and laser micro-processing. Another preferred method to form theferrule mold is by using SU-8 photopolymer.

Electroplating

An electroplating process to form the ferrule chip is conducted bymaking electrical contact to the metal layer 104 in an electroplatingbath, to gradually fill up the empty space of the silicon ferrule moldwith metal 320 from the metal layer 104, until it overflows from the topof the silicon pillars. FIG. 2i is a schematic view of the ferrule moldfilled with metal to form the ferrule chip after electroplating iscompleted.

A preferred material for metal 320 may be a nickel alloy. However, othermaterials having over 50 GPa Young's modulus can be used. The moldfilling method is not limited to electroplating. For example, stampingor nanoimprint methods may also be used to create the metal ferrule chip20.

Other deposition methods may also be selected depending upon thematerial to be used to form the ferrule. These include vapor deposition(chemical vapor deposition, evaporation, sputtering, ion beamdeposition) or molding techniques when ceramic-based materials areselected for the ferrule.

Polishing to Remove Excess Metal

The electroplated wafer of FIG. 2i is polished along polishing plane 322to remove the over-deposited metal, ensuring a flat surface of thefabricated ferrule chip. FIG. 2j is a schematic view of the ferrule chipstill in the ferrule mold after the polishing step is completed.

Ferrule Mold Release

Metal ferrule chips 20 are released from the silicon ferrule mold byetching away the silicon wafers by a suitable process, for example wetetching. Mechanical methods of mold release may also be selected. Dicingmay be unnecessary because the ferrule chips are separated by ferrulechip wall 314. FIG. 2k is the finished ferrule chip 20.

Ferrule Chip Configuration

FIG. 2l is the ferrule chip 20 in further detail. The ferrule chip 20 isa thin layer of metal material. It has two sets of through-holes,including two guide holes 330 and five fiber through-holes 332. Eachguide hole 330 is dimensioned and configured to hold and align a guidepin, while each fiber hole 332 is dimensioned and configured to hold andalign an optical fiber.

In the preferred embodiment the ferrule chip 20 has a thickness ofapproximately 700 microns. This thickness provides sufficient structuralintegrity, while being thin enough to be made economically using amicrofabrication process.

While a 25-count array of fiber holes is depicted for clarity ofpresentation (5 in cross section times five rows deep), it is understoodthat substantially larger numbers of fiber holes in arbitraryconfigurations may be formed in the multi-fiber connector ferrule of thepresent invention. For example, a 2D array of 12×12 fiber holes or 24×24fiber holes may be made using the method of the present invention.

Tilt Angle

As shown in FIG. 2l , the central axes 340 of guide holes 330 andcentral axes 342 of fiber holes 332 should be parallel to each other. Inone embodiment, there is a small tilt angle 350 between the hole axis340 and the surface normal 344 of the ferrule chip 20. This tilt anglemay help to improve the multi-fiber optical connector's return loss.This tilt angle may be preferably from 5-8 degree, but may be otherangles, for example, from 1 degree to 15 degree.

Fiber Hole and Guide Hole Profile

As shown in FIG. 2l , near top surface 334 of the ferrule chip 20, thefiber hole and guide hole diameters are uniform along the central axis340 and 342 to provide accurate fiber angular alignment, but becomewider to facilitate easy fiber insertion from the back side 336 of theferrule chip 20.

Fiber holes and guide holes are most accurate on top surface 334 of theferrule chip 20, where guide holes 330 and fiber holes 332 havesub-micron accuracy in their central locations due to themicrofabrication process. This accuracy ensures accurate alignment of SMfibers with minimum insertion loss.

Combining Ferrule Chip with Ferrule Pedestal

FIG. 3a shows a multi-fiber connector ferrule 10, including a ferrulechip 20 and a ferrule pedestal 30. Because the ferrule chip 20 is a thinlayer, the multi-fiber connector ferrule 10 optionally uses additionalstructure during later processing and in eventual operation inside anoptical fiber connector housing. The ferrule chip 20 may be affixedpermanently to a ferrule pedestal 30 to form the multi-fiber connectorferrule 10.

The ferrule pedestal 30 may include a flange 402, a fiber cavity 404,two guide pin cavities 406 for fibers and guide pins to pass through,respectively. Fiber cavity 404 is preferably separate from the two guidepin cavities 406. By separating the fiber cavity, epoxy used to affixthe fibers does not flow into and block the guide holes 330.

Guide pin cavities 406 may have a slightly larger diameter than thediameter of the guide hole 330 near the ferrule top surface 334, toavoid interference with the operation of the guide holes 330. However,the guide pin cavities 406 may have a narrow part near their bottomportions, to define the angle of the guide pins (two points determine aline).

The ferrule pedestal 30 may have a flange 402 for easy positioning andsurface registration. The ferrule pedestal 30 may have a tilted surface408, and the angle of the tilted surface is the same as tilt angle 350of the ferrule chip 20 in FIG. 2l , so that fiber hole axis 342 isperpendicular to the flange surface 402. The flange design facilitates asubsequent polishing operation, and when the multi-fiber connectorferrule 10 is assembled into a connector housing, not shown. The ferrulepedestal 30 may be formed from plastics, metal, or any other rigidmaterial.

Fiber Insertion

FIG. 3b depicts a cross-sectional view of the multi-fiber connectorferrule 10, after fibers 412 are inserted through the fiber holes andpermanently affixed with epoxy or other suitable fixing agent. Theoptical fiber 412 may be a SM or multimode optical fiber. A length offiber with plastic jacket stripped 414 is shown in FIG. 3 b.

Care must be taken to ensure that no epoxy flows into the guide holes330. Care must be taken to ensure that epoxy does not flow downwardalong the fibers 412 through a capillary effect and make the fibers 412too rigid. Otherwise it would require too much force on the guide holes330 to passively align the connector ferrule 10.

As shown in FIG. 3b , after affixing the fibers with epoxy, the fibers414 protrude from the ferrule chip surface 334.

The multi-fiber connector ferrule 10 is polished along polishing plane420, which is parallel to original ferrule chip top surface 334. Thisensures that very little ferrule chip material is polished away.

Because fiber holes and guide holes are most accurate on top surface 334of the ferrule chip 20, and plane 420 is very close to plane 334, fiberholes and guide holes are very accurate on plane 420 as well.

Recessed Fiber Endfaces

The multi-fiber connector ferrule 10 should have fiber endfaces 418slightly recessed from the new ferrule chip top surface 420, as shown inFIG. 3 c.

To achieve the recessed fiber endfaces 418, one embodiment uses adifferential polishing process. This differential polishing process usescerium oxide as the final polishing particle. During differentialpolishing, the glass material of the optical fiber is removed at agreater rate than the nickel alloy material of ferrule chip 20.Therefore, the fiber endfaces 418 are recessed with respect to thesurrounding ferrule chip top surface 420.

Fiber endfaces 418 are prevented from making contact with opposing fiberendfaces in a mating connector by the recessed fiber endfaces. For SMfiber, a recess of approximately 0.5-1.0 micron is sufficient to preventfiber endfaces 418 from making contact with mating fiber endfaces.

Recessed fiber depth can be made larger (3-5 microns for example) sothat any large contaminants such as dust would be contained in therecess without affecting fiber endfaces 418. This larger recess makesthe multi-fiber connector more tolerant of dusty and contaminatedenvironments. Recessed fiber depth from 0.1 micron to 10 microns may beselected.

An alternative method to achieve recessed fiber endface 418 is todeposit a thin layer of spacer material on the top surface 420 of theferrule chip 20. One preferred embodiment may be additionalelectroplating after fibers are polished, to plate a thin layer of metalsuch as nickel on the metal surface of the ferrule chip 420, causing thefiber endfaces 418 to be recessed from the surrounding ferrule chipsurface. Because the silica fiber is non-conducting, electroplatednickel will not cover the fiber endfaces 418 and therefore a spacer isformed.

AR Coating

In order to eliminate the multiple reflections between two fiberendfaces with an air gap in between, an antireflection (AR) coatinglayer 424 is coated over the fiber endfaces 418 as shown in FIG. 3d .Typically the top surface 420 of the ferrule chip 20 is also coated. TheAR coating layer 424 is intended to cover the entire surface, exceptguide holes 330 which are protected by a suitable deposition mask.

The AR coating band is selected according to the operating wavelengthrange of the optical system in which the connector is to be used. An ARcoating with residual reflectivity of less than 0.2% may be selected.The thickness of the AR coating is typically on the order of 1 micron.This thickness is sufficient to achieve the desired antireflectionproperties.

Male Ferrule Chip

FIG. 3e shows a cross-sectional view of the multi-fiber connectorferrule 10 with guide pins 426 in place to create a “male ferrule”. Aperspective view of the same part is shown in FIG. 3f . Guide pins 426typically have a chamfered tip which facilitates insertion into theguide holes. Guide pins 426 may be made of stainless steel with aprecise diameter and smooth surface. However, other rigid materials suchas ceramics may also be used to make the guide pins 426.

Two Ferrules Mated Together

FIG. 3g depicts a cross-sectional view of female and male multi-fiberconnector ferrules mated together. The AR coating layers 424 of the twoopposing connector ferrules contact each other.

Guide pins 426 are positioned by the guide holes to align the topconnector ferrule to the bottom connector ferrule, thereby achievingprecise sub-micron alignment of each of the fibers 412 with matingfibers of the opposing connector ferrule. The presence of fiber recesswill prevent the fiber endfaces from making contact. The AR coating 424on the surface of the ferrule chip 20 will make contact during theoperation of the connector, but because the connector ferrule surface isflat and large, the force is distributed over a larger area, preventingflaking or chipping of AR coating 424.

Optional Tilt Angle

The reasons for the optionally tilted fiber holes and guide holes arethe following. It is well known that fiber connector endfaces are oftenpolished at an angle (for example 8 degree) in order to have a highreturn loss value, for example, above 60 dB. If fiber holes and guideholes are formed in a new ferrule chip 440 with a tilt angle of zerodegree, as illustrated in FIGS. 4a and 4b , then the new ferrule chip440 may optionally be polished with a tilt angle of 8 degree, forexample.

FIG. 4a shows the new ferrule chip 440 with fibers 412 inserted andaffixed with epoxy. Polishing plane 450 is shown. Fiber holes and guideholes are most accurate on the top surface 460 of the ferrule chip 440.

FIG. 4b shows the female multi-fiber connector ferrule after thepolishing step, ready to receive guide pins 426. Two consequences can beseen:

An 8-degree polishing step could polish away one side of the ferrulechip 440 completely, because the ferrule chip is only about 0.7 mm thickin the preferred embodiment.

After a polishing step, guide hole openings may not be accurate enoughto align guide pins 426, because the most accurate part of the guideholes on the top surface 460 of FIG. 4a may be polished away, resultingin enlarged guide hole openings. Likewise, fiber endface positions maynot be sufficiently accurate.

The major benefit of the tilt angle is that grinding away of significantferrule chip material is avoided. Referring to FIG. 3c , all fiber holeopenings and guide hole openings have the same shape after a polishingstep. Fiber holes and guide holes are the most accurate, because theyare polished very little.

In a preferred embodiment for high return loss, the tilt angle is chosento be 6 degree or 8 degree, but it can be any angle from 1 degree to 15degree. If high return loss is not a concern, a tilt angle of zerodegree may also be selected.

Optical Performance of the Multi-Fiber Connector

Because the multi-fiber connectors disclosed here are formed byphotolithographic processes, the center positions of the guide holes andfiber holes have tolerances of about 0.1 micron, and the diameters ofthe guide holes and fiber holes can be controlled to have a variation ofless than 1 micron.

In terms of optical performance, SM multi-fiber connectors made with themethod disclosed herein can have insertion loss range from 0 to 0.3 dB,and return loss from 55 to 80 dB. This is approximately the sameinsertion loss as a SM optical fiber connector (LC, SC, FC etc.,) andthus is acceptable in the “loss budget” of an optical system. Thefabrication techniques are reproducible and scalable to mass-productionof multi-fiber connectors.

Metal ferrules made of materials such as nickel have many advantagescompared to molded plastic MT ferrules.

A metal connector ferrule has much greater hardness, much lowercoefficient of thermal expansion (CTE), much better thermal stability,can withstand a much higher processing temperature, will outgas muchless in a vacuum environment such as an AR coating chamber, has muchbetter resistance to guide pin abrasion and much longer mating lifetime,and has smaller electromagnetic interference due to the conductive metalmaterial.

Photomask Pattern Elongation Due to Tilt Angle

Referring to FIG. 2l , because the tilted fiber holes and guide holesare sized to receive round fibers and round guide pins, the crosssection of fiber holes and guide holes must be round on a planeperpendicular to the fiber axis. A 2D array of round fiber hole patternswith equal pitch in the two directions on plane 338 will become a 2Darray of elliptical fiber hole patterns with an unequal pitch whenprojected on the ferrule chip surface 334, the latter of which are thepatterns that the photomask must be designed with. The length of thephotomask patterns is “stretched” in one direction with a scaling factorof 1/cos(θ).

Preferably, the tilt angle θ should be in one direction of the 2D fiberhole array, although this is not necessary.

If the tilt angle θ is 6 degree, the scaling factor is 1.01. This is asmall correction; however, it ensures sub-micron alignment of fibers inthe fiber holes. For example, a round hole of 126 micron diameter onplane 334 will become an elliptical hole of sizes 126 micron and 124.7micron on plane 338, with the smaller number being too small for a 125micron diameter fiber.

In order to form a 2D array of fiber holes of 126 micron diameter with250 micron pitch with a tilt angle of 6 degree, a photomask may bedesigned to have a 2D array of elliptical holes of sizes 126 micron and127.26 micron with a pitch of 250 micron and 252.5 micron in the twoorthogonal directions.

Dust Channel in the Guide Hole

Because the manufacturing method disclosed herein uses microfabrication,arbitrarily-shaped guide holes may be formed. Various modifications tothe guide hole may be made.

In conventional MPO connectors, guide holes are made by molding plasticsaround a cylindrical object; therefore, it is nearly impossible to haveany other shape for the guide holes than round guide holes. Dust orcontaminants tend to build up and clog the micron-sized gap between theround guide pin and round guide hole.

FIG. 5 shows top view of a ferrule chip with modified guide holes 500,which has original unmodified sidewall parts 502 which contact the roundguide pin, and modified sidewall parts with “dust channels” 504 which donot contact the round guide pin. The guide hole's dust channels 504 runvertically along the side walls. Alignment of the guide pin is providedby the unmodified sidewall 502. With this structure, small amount ofdust or contamination on the guide pins is pushed into the dust channelsof the guide holes during connector mating. Therefore, enhanced dusttolerance of this connector is achieved without reducing the alignmentaccuracy of the multi-fiber connector ferrule and without sacrificingany optical performance.

Spring-Loaded Guide Hole

FIG. 6 shows top view of a ferrule chip with modified guide holes 600which has spring-loaded sidewalls. By forming hollow channels 602, partof the sidewall 604 of the guide hole has a thin, deformable wall. Thedeformable wall acts as a spring-like structure in deforming andresuming its original shape and thus can act as a spring-loaded guidehole.

There are two benefits to a spring-loaded guide hole 600. First, guidepins slightly larger than the undeformed guide hole diameter can beinserted when the spring-loaded wall structure deforms, leaving zero gapbetween the guide pin and guide hole, and a more precise self-centeredalignment. Second, due to the adjustable nature of the spring-loadedguide hole, the guide pin does not need to have the same diametertolerance that a fixed guide hole would require. The cost of the guidepins is reduced as a result.

It is important to ensure the hollow channels 602 are not filled withdebris. Otherwise the spring-loaded guide hole 600 may be affected.

Elongated Female Guide Hole

Referring to FIG. 3g , the distance between the two guide holes has atolerance of less than 1 micron. However, at the tips of the guide pinsof the multi-fiber connector ferrule, the distance between the two guidepins is frequently not exactly the same as that of the guide holedistance, due to the guide pins being not parallel to each other, or dueto slight bending of the guide pins. When the two guide pins first enterthe two female guide holes, this mismatch will cause excessive abrasionof the guide pins and guide holes. The entrance of the guide holes tendsto be eroded quickly, resulting in the loss of precision of guide holealignment, and the reduction of the mating life of the multi-fiberconnector.

To greatly reduce the above mismatch problem, FIG. 7 shows top view of afemale ferrule chip with modified guide holes, where one guide hole 704is round while another guide hole 700 has an elongated profile in thedirection 702. The round guide hole 704 serves as a position-defininghole, and the elongated guide hole 700 serves as a rotation-limitinghole.

This ferrule chip is useful as a female ferrule chip. A male ferrulechip should still have two round guide holes.

This design provides relief for the mismatch mentioned above, withoutreducing the alignment accuracy of the multi-fiber connector pair. Theguide pins of the male multi-fiber connector ferrule can be insertedinto the guide holes of the female multi-fiber connector ferrule even ata slight angle.

This design would have been very difficult to implement using plasticmolding processes by which traditional MT ferrules are made. However,because of the microfabrication process disclosed here, arbitrary shapedguide holes can be formed at will, with great positional accuracy.

Alternate Recessed Fiber Connectors

With reference to FIG. 8, an embodiment of the non-contact optical fiberconnector component according to the present invention is a non-contactfiber ferrule assembly for making non-contact optical fiber connectors.An optical fiber 1020 is permanently affixed in the axial through hole1025 of a connector ferrule 1010 with epoxy, and a metal flange 1015 isconnected to the ferrule 1010. The front surface of the ferrule 1017forms a smooth polished, curved profile with the fiber surface 1013somewhat offset from surface 1017. An AR coating 1040 is applied overthe entire polished surface of the ferrule 1017 and the fiber facet1013. The fiber 1020 can be any type of optical fiber. For example, itcan be single mode fiber, multimode fiber, or polarization maintainingfiber.

FIG. 9 shows a pair of such non-contact fiber connector componentscoupled together to complete a fiber connection with the aid of analignment split sleeve 1150 found in a connector adapter. A conventionalfiber connector adapter is used to align the two non-contact fiberconnectors. The two ferrules 1010 and 1110 are shown precisely alignedby a split sleeve 1150 which sits at the center of a fiber connectoradapter. A first fiber 1020 communicates light to a second fiber 1120through a gap 1121 that exists between the two fibers by virtue of thefibers being slightly recessed. Thus, while the AR coatings 1040 and1140 on the front surfaces of ferrules 10 and 1110 are in contact, theAR coatings on the fiber facets are not in contact. Therefore, thisfiber optic connector is called a non-contact connector.

We now describe the non-contact fiber connector component in FIG. 8 inmore detail, in the order of the manufacturing sequence. The non-contactoptical fiber connector component of FIG. 8 includes a ferrule 1010 thatis a conventional connector ceramic ferrule, typically a zirconiaceramic tube having a standard length and diameter. Most often theferrule 1010 has a length on the order of 0.5 to 1.3 cm, and thediameter may be 2.5 mm or 1.25 mm. The ferrule 1010 has a polished frontend 1017 and a rear end 1019. In turn, the rearward portion of ferrule1010 is connected to a metal flange sleeve 1015, being permanentlyaffixed to ferrule 1010 with a tight press fit. Glass fiber 1020 isinserted into the coaxial ferrule inner hole 1025 and permanentlyaffixed by epoxy (not shown). Protected fiber cable 1030 is rearward ofthe ferrule 1010.

The fiber ferrule assemblies are then polished at the light output endso as to render a smooth surface 1017 on the ferrule 1010. The polishangle, measured as tilt from vertical at the fiber core, where verticalis perpendicular to the fiber axis, can be zero degrees, or non-zerodegrees to minimize back reflection. In a preferred embodiment, thepolish angle is 8 degrees. Just as in conventional fiber connectorswhere the connector ferrule surface is a convex surface, ferrule frontsurface 1017 should be convex as well.

Differential Polishing

The polishing process for non-contact fiber connectors in this inventionis very similar to conventional connector polishing, except the finalpolishing step. After a fiber stub removal step, a series ofprogressively finer lapping films are used to polish the connectorsurface, typically from 9 microns, 3 microns, to 1 micron diamondparticles. Final polish step is then performed.

The final polishing step in this invention is different fromconventional connector polishing, and is the step responsible forforming the recess in the fiber. In this step, the fiber ispreferentially and differentially polished relative to the ferrule frontsurface so as to create a recess between the fiber facet 1013 andferrule front face 1017. The recess range should be kept as small aspossible to reduce optical coupling loss, while ensuring no physicalcontact between the opposing fiber facets when mated.

For a single mode fiber SMF-28, the light beam is best described as aGaussian beam. In air, the working distance (Rayleigh range) is about100 microns. If the fiber recess is 0.5 micron, light from the fibercore traveling twice the recess length does not expand sufficiently toinduce significant optical coupling loss. The extent of a recess ispreferably in the range of 0.1 microns to several microns, moreparticularly, 0.1 micron to 0.5 micron, 0.1 to 1.0 micron, or 0.1 micronto 10 microns.

The recessed fiber facet 1013 in FIG. 8 can be created by polishing withflocked lapping films. These are lapping films with micro brushes whichhave abrasive particles embedded in them. For example, 3M flockedlapping film 591 can be used to create this recess. This is a lappingfilm with micro brushes which have 0.5 micron cerium oxide particlesembedded in. Cerium oxide has a hardness very similar to that of theoptical fiber but much softer than the zirconia ceramic ferrule 1010,and as a result, only the fiber surface 1013 is polished in this step.This step generates a very smooth optical fiber surface and typically isthe last polishing step. The time in the final polishing step varies,and can be as short as 20 seconds. Polishing pressure in this final stepshould be kept lower than the previous polishing steps, in order toextend the lifetime of the flocked lapping film. Flocked lapping filmswith other polishing particles can be used as well, such as aluminumoxide or silicon nitride.

Finally, an AR coating 1040 is applied to the polished surface of thefiber 1013 and front surface of the ferrule 1017. The operatingwavelength range of the AR coating determines the operating wavelengthrange of the non-contact optical fiber connector in this invention.

In a preferred embodiment, many polished fiber ferrule assemblies areloaded into a vacuum coating chamber and coated with a multi-layer stackof dielectric materials. Numerous AR coating processes can be used. Forexample, the coating method can be ion beam sputtering or ion-assistede-beam deposition. Care should be taken to prevent significant amount ofthe coating material from getting on the sidewall of the ferrulecylindrical surface, by suitable masking. Otherwise the material willalter the precision diameter of the ferrule, and cause flaking off ofcoating material which will affect connector performance.

The fiber cables to be coated in an AR coating chamber must not outgassignificantly in a vacuum chamber. We have observed that the inclusionof a mere ten 0.9 mm loose tube buffered cables in the chamber canlengthen the vacuum pumping time from 2 hours to more than ten hours forion beam sputtering. The materials of the fiber cable must be chosencarefully to reduce outgassing. Bare fibers housed in ferrules in the ARcoating chamber are optimal.

FIGS. 10(A) and 10(B) are contour plots of the recessed fiber surfacesof the non-contact fiber connector, polished by a 0.5 micron ceriumoxide flocked lapping film, as measured by a commercial fiber opticinterferometer. To show the recessed fiber surface, the connectorsurface was tilted intentionally in order to show continuous heightcontours. Different amounts of polishing time were used in these twocases. The depth of fiber recess in the plots was estimated to be 0.5micron and 2.8 micron respectively. Some curvature on the fiber surfacecenter can be seen from these two plots, but the amount of curvature isnot large enough to significantly alter light beam propagation betweenthe recessed fiber facets.

We have polished more than 500 non-contact fiber connectors with zeroscratches, which is very different from the final polish step ofconventional connectors where scratches are frequent and inspection andrepolishing are required. As a result, 100% inspection of connectorpolishing after final polish step becomes unnecessary which can savesignificant manual labor cost.

Non-Contact Fiber Connector Performance

Several hundred non-contact fiber connectors with recessed fiber facetshave been made to date with great manufacturing yield. Both zero degreeand 8° angled non-contact (ANC) single mode fiber connectors were made.

The insertion loss of both zero degree and 8° ANC connectors showsnearly identical loss distribution to that of conventional fiberconnectors, for example with a range of 0-0.2 dB insertion loss. Theinsertion loss in all three cases is dominated by the errors in thefiber core positions due to geometrical tolerances.

A mated pair of zero degree NC connectors has about 30 dB return loss,while a mated pair of 8 degree ANC connectors has more than 70 dB returnloss, or about 10 dB higher return loss than conventional 8 degree APCconnectors.

Both NC and ANC connectors have essentially guaranteed insertion lossperformance in random mating. Therefore, an ANC connector is thepreferred connector because it has superior return loss performance.

We have tested a pair of ANC connectors and found it lasted through10,000 matings with less than 0.01 dB insertion loss change from thebeginning of the test to the end.

The non-contact fiber connector of the type shown in FIG. 8 greatlyimproves the optical performance and the durability of the fiberconnector and meets the needs of most applications.

FIG. 11 is a cross sectional view showing another embodiment of thenon-contact optical fiber connector component according to the presentinvention. Another means for providing a recess of the fiber facetrelative to the ferrule front surface is to coat the ferrule surfaceselectively with a metal coating 1045 as a spacer layer on top of the ARcoating layer 1040. Metal coatings having a thickness of from a fractionof a micron to a few microns (for example 10 micron) may be applied byvapor deposition or ion beam sputtering using techniques known in thesemiconductor industry. Such coatings are known to be resistant to wearand tear.

In this embodiment, the fiber ferrule assembly can be polished using aconventional connector polishing process. The result of this polishingprocess is that the fiber is at the apex of the convex surface. Thepolishing angle can be zero degrees or 8 degrees. The metal coating canbe accomplished by a suitable masking operation so that the metal doesnot cover the fiber surface. Note that the AR coating 1040 covers boththe output facet 1013 of the fiber 1020 and the front surface 1017 offerrule 1010.

In conventional connector cables, frequently a long length of reinforcedfiber cable is used between two optical fiber connectors. For example,one of the most used fiber cables is a 3 mm diameter cable with Kevlarfabric reinforcement. Such a cable will outgas greatly in a vacuumchamber, occupy too much room and difficult to manage inside the ARcoating chamber. Clearly AR coating entire fiber connector cables in anAR coating chamber is not an option.

Instead, only the most essential part of the connector with very shortlength fiber should be loaded in. After AR coating, such short fibershould be connected to the long reinforced cable by fusion splicing,which is a very reliable and relatively low cost fiber connectionmethod.

Splice-on connectors are known in the prior art. These are conventionalconnectors that have factory-polished connector surfaces with a shortlength of cleaved fiber at the rear of the connector head ready forfusion splicing to a long length of typically reinforced fiber cable.

FIG. 12 is a schematic drawing of a generic non-contact optical fiberconnector with a splice-on connector construction. This construction isa necessary part of the low cost mass production process, because itallows non-contact fiber connectors to have very long fiber cables andreinforced fiber cables. The splice-on structure of the coupling devicealso allows non-contact fiber connectors to be installed in the field.

In FIG. 12, a non-contact fiber ferrule assembly is housed in aconnector structure, which comprises a housing 1550, a spring 1535, amain body 1580, a rubber boot 1590. The spring 1535 provides positiveforce to the fiber ferrule 1510, which has a fiber 1520 inside itsthrough hole. An AR coating 1540 is at the front surface of the fiberferrule assembly and covers the fiber facet. The fiber at the rear ofthe fiber ferrule 1510 has a protected bare fiber section 1530. It isstripped and cleaved to expose a glass fiber section 1560. A long fibercable 1595 is stripped and cleaved to expose a glass fiber section 1575.These two glass fiber sections are fusion spliced together at fusionsplicing joint 1570. The glass fiber sections should be as short aspossible, so that the splice-on connector is not too bulky. Each glassfiber section is preferably 5 mm in length. Because the fusion splicedjoint is very weak, it is reinforced by a conventional fusion splicingprotection sleeve 1565, which is attached at one end of the metal flange1515 and at the other end to long cable 1595. There is a steel rodinside the protection sleeve to give it strength.

FIG. 13 is a schematic drawing of a sample holder 1620 for AR coating avery large number of fiber ferrule assemblies simultaneously. The holder1620 is machined with many closely spaced, ferrule sized holes 1630 sothat a large number of fully polished fiber ferrule assemblies 1610 ofthe type depicted in FIG. 8, without the AR coating, may fit in.Thousands of such assemblies can be AR coated in the same coating runusing such a holder 1620 to reduce manufacturing cost.

The non-contact fiber connector operating principle established abovecan be used for multi-fiber connectors as well, such as MT type arrayconnectors. FIG. 14 is a plan view of a non-contact multi-fiberconnector pair according to an embodiment of this invention. A pluralityof optical fibers 1750 are permanently affixed (this is a ribbonassembly with no structural coating) in the axial through holes of themulti-fiber connector ferrule block 1710 with epoxy. As seen in FIG. 14,optical fibers 1750 are in the form of a ribbon fiber cable. The frontsurface of the ferrule block 1710 forms a smooth polished profile withthe fiber facets 1720 recessed. An AR coating is applied over the entirepolished front surface of the ferrule block 1710 and the fiber facets1720.

When a multi-fiber connection is made using two non-contact multi-fiberconnectors as in FIG. 14, two guide pins 1740 go through one ferruleblock 1710 and enter the precisely formed guide holes 1730 of theopposing ferrule block to align the two multi-fiber connectors. Thepolished front surfaces of the two multi-fiber connectors must makecontact due to the springs in the connectors (not shown). A latch, notshown, holds the two ferrule blocks 1710 together. Due to the fiberfacets being recessed, the fiber facets do not touch, resulting inreliable and long lasting operation of the non-contact multi-fiberconnector.

Fiber facets 1720 can be offset from ferrule block front surface by anumber of means. Selective etching, differential polishing, metaldeposition, or simply deforming the polished ferrule surface can allachieve non-contact of fiber facets. In all cases, small gaps betweenfacing fibers can communicate optical signals from fiber cables tomating cables. The facets can have a slight angle, say 8 degrees.

It will be appreciated by those skilled in the art, in view of theseteachings, that alternative embodiments may be implemented withoutdeviating from the spirit or scope of the invention, as set forth in theappended claims. This invention is to be limited only by the followingclaims, which include all such embodiments and modifications when viewedin conjunction with the above specification and accompanying drawings.

The invention claimed is:
 1. A polishing method of making a recessedfiber, non-contact, multi-fiber optical connector at an end of anoptical fiber cable, the recessed fiber non-contact, multi-fiber opticalconnector having plural optical fibers positioned in a ferrule block,comprising: providing an optical fiber ferrule block, the ferrule blockincluding guide pins or guide holes to align multi-fiber connectors;inserting plural optical fibers in separate through-holes in the ferruleblock, the ferrule block having a ferrule block front surface, eachthrough-hole being sized to accept and align an optical fiber, each ofthe plural optical fibers being part of a multi-fiber optical fibercable that does not significantly outgas in a vacuum deposition chamber;affixing the plural optical fibers in the ferrule to form afiber-ferrule-cable assembly; performing differential polishing of theplural optical fibers and the ferrule block front surface, so that asmooth fiber facet and a ferrule block contact surface are formed, and arecess between a fiber facet and the ferrule block contact surface isformed such that the created recess ranges from 0.1 micron to 10microns; applying a multi-layer dielectric antireflection coating usinga method selected from ion beam sputtering or ion-assisted e-beamdeposition under a vacuum deposition condition to the recessed fiberfacet and to the ferrule block contact surface such that the multi-layerdielectric antireflection coating, after application, is a two-levelantireflection coating having a first coating level on the front surfaceof the ferrule block and a second coating level on the polished surfaceof the plural optical fibers, the first coating level being higher thanthe second coating level such that when the first coating levelmulti-layer dielectric antireflection coating on the front surface ofthe ferrule block contacts another ferrule block during connectormating, the second coating level on the plural optical fibers does notcontact mating fibers to form a multi-fiber non-contact opticalconnector.
 2. The polishing method of making a recessed fiber,non-contact multi-fiber optical connector according to claim 1, whereinthe differential polishing uses a series of progressively finer lappingfilms to perform the polishing.
 3. The polishing method of making arecessed fiber, non-contact multi-fiber optical connector according toclaim 2, wherein one or more lapping films includes micro brushes withparticles embedded therein.
 4. The polishing method of making a recessedfiber, non-contact multi-fiber optical connector according to claim 3,wherein said particles have a hardness approximately equal to thehardness of the optical fiber and softer than a material of the ferrule.5. The polishing method of making a recessed fiber, non-contactmulti-fiber optical connector according to claim 3, wherein theparticles are selected from cerium oxide, aluminum oxide, or siliconnitride.
 6. The polishing method of making a recessed fiber, non-contactmulti-fiber optical connector according to claim 1, wherein a polishangle is a non-zero degree.
 7. The polishing method of making a recessedfiber, non-contact multi-fiber optical connector according to claim 1,wherein a polish angle is approximately eight degrees.
 8. The polishingmethod of making a recessed fiber, non-contact multi-fiber opticalconnector according to claim 1, wherein said optical fiber cable thatdoes not significantly outgas in a vacuum deposition chamber is a ribbonfiber cable.